Listeria innocua

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A Microbial Biorealm page on the genus Listeria innocua


Higher order taxa:

Bacteria; Firmicutes; Bacilli; Bacillales; Listeriaceae; Listeria


Listeria innocua

Description and significance

Listeria innocua is one of the six species belonging to the genus Listeria. It is widely found in the environment (such as soil) and food sources. It can survive in extreme pH and temperature, and high salt concentration (5). In terms of appearance, it is a rod-shaped Gram-positive bacterium. It is a non-spore forming bacterium. It may live individually or organize into chains with other Listeria innocua bacteria. It is a mesophile, operating at an optimal temperature range of 30-37 degrees Celsius. Listeria innocua very much resembles its other family members, the pathogenic Listeria monocytogenes (5). Listeria innocua was isolated from meat by a technique called surface adhesion immunofluorescence (3). Samples of meat were inoculated with L. innocua and incubated at 30 degrees Celsius for 14-18 hours in a broth. The cells were then isolated from the meat by surface adhesion onto a polycarbonate membrane attached to a microscope glass slide. Then immunofluorescent microscopy was employed to visualize the bacteria (3). Listeria innocua is important because it is very similar to the food-borne pathogen L. monocytogenes but non-pathogenic in character. Thus its genome was sequenced in order to compare it to the genome of L. monocytogenes to learn what makes the latter pathogenic.

Genome structure

Listeria innocua has a circular chromosome made up of 3,011,209 base pairs, with a 37% G+C content (6). Only 2,973 protein-coding genes were discovered in L. innocua, while no function could be predicted for 37% of the genes (6). The genome encodes a great variety of regulatory, surface and transport proteins (5). This explains why Listeria species can adapt to and inhabit such a wide range of environments. Listeria innocua was found to be deficient in a 10-kb virulence locus, a cluster of genes that engenders pathogenicity to Listeria monocytogenes (5). This explains why L. innocua typically does not infect humans or animals. It also has a circular plasmid of 81,905 base pairs whose function is unknown (6).

Cell structure and metabolism

Listeria innocua are Gram-positive bacteria, meaning they have a thick cell wall for protection. The cell wall is studded with hydrophilic molecules such as teichoic acids to repel hydrophobic molecules such as drugs. These organisms also produce flagella and pili for motility. Listeria innocua have very complex metabolism. They are capable of metabolizing methane, sulfur and nitrogen, among many other organic and inorganic compounds (4). These organisms also carry out numerous biosynthetic pathways, including peptidoglycan synthesis. Listeria innocua, like other members of their genus, are facultative anaerobes, which means that they can metabolism glucose (and other simple sugars) in under both aerobic and anaerobic conditions. Under the aerobic metabolism of glucose, L. innocua forms lactic acid and acetic acid. However, under anaerobic conditions, the metabolism of glucose yields only lactic acid (4).


Listeria species are unusual in that they can survive and multiply at both low and high temperatures. They can also endure a wide pH range of 4.4 to 9.8 (4). These facts help explain why they are ubiquitous in all types of environment. Listeria spp. are found in water, soil, vegetation, wild and domesticated animals, and humans. Cultivated land has less contamination of Listeria than uncultivated; sandy soil contains more Listeria than clay-like soil; untreated sewage water is a good breeding ground for Listeria. Since Listeria is also found in water sources, such as the sea, it is not surprising that fish, squids, crustaceans and other seafood have been found to contain the bacteria (8). This could be harmful for humans if the contamination is from L. monocytogenes because they are the pathogenic species that cause disease (5). Furthermore, human intervention, like effluents from food-processing plants, also increases the spread of Listeria into the environment (8).


Listeria innocua, as its name suggests, is harmless to other organisms. It lacks the 10-kb virulence locus that is needed for pathogenicity. Out of the six species of Listeria, only L. monocytogenes causes a disease called listeriosis in both human and animal hosts (8). Listeriosis has a high fatality rate and ranks among the most frequent causes of death due to food poisoning. The ability of L. monocytogenes to resist food-processing conditions and refrigeration temperatures makes it a threat to public health via contamination of meat, poultry, seafood and dairy products (8).

Infection occurs in several detailed steps. First, the bacterium enters the host cell. Second, it escapes from the host’s phagosomal vacuole. Third, it multiplies in the cytosol. Finally, it spreads to other cells using actin-based motility. Each step in the infection process requires specific enzymes expressed by virulence factors located on the bacterium’s chromosome (8). For example, internaline genes encode internaline proteins which are responsible for the invasion of host epithelial cells; the gene hlyA encodes listeriolysin O and the gene plcA encodes PI-PLC, both of which are involved in the lysis of the phagosome (8).

Some symptoms associated with listeriosis include fever, muscle aches, diarrhea, vomiting and nausea. If the infection has spread to the brain or spinal column, additional symptoms include confusion, stiff neck, headache and loss of balance. Pregnant women, neonates, elderly people and people with depressed immune systems are most susceptible to listeriosis, which manifests as septicaemia, meningitis, or meningoencephalitis. Moreover, in pregnant women, the disease can cause spontaneous abortion (8).

Application to Biotechnology

Recently, researchers discovered that four isolates of Listeria spp. produced inhibitory activities against the pathogen L. monocytogenes. One of the isolates, Listeria innocua 743, was “selected for further study” (2). It was revealed that the plasmid of L. innocua produced two compounds: a bacteriosin and a protein “involved in immunity toward other bacteriosins” (2). A bacteriosin is a type of antibiotic. It is a small peptide molecule with 30-60 amino acid residues. Bacteriosins are commonly produced by lactic acid bacteria and other bacterial species, but this is the first known case of Listeria spp. producing bacteriosin. This discovery has beneficial implications for controlling Listeria monocytogenes, which presents a danger as a food-borne pathogen. Bacteriosin can be used to inhibit the growth of L. monocytogenes in processed foods, thereby making the ingestion of such foods much safer for humans. Furthermore, the bacteriosin produced by Listeria innocua 743 revealed a very broad spectrum inhibition of L. monocytogenes, which means that the antibiotic can inhibit a wide variety of L. monocytogenes strains (2).

Current Research

1) A current research conducted by the Department of Veterinary and Microbiological Sciences at North Dakota State University “examined the antimicrobial susceptibility of 86 Listeria spp. isolated from processed bison carcasses” (9). Testing 25 antimicrobial agents on the isolates, it was ascertained that most (88-98%) exhibited resistance to bacitracin, oxacillin, cefotaxime, and fosfomycin (9). Tetracyline resistant was also common among the Listeria isolates (18.6%) (9). Furthermore, the scientists discovered that there were “differences in resistance among Listeria spp.,” with L. innocua showing the most resistance to the antibiotics and L. monocytogenes showing the least resistance (9). The significance of this fact is that the high antimicrobial resistance displayed in L. innocua has the potential to transfer resistance to the low-resistance L. monocytogenes. This could be detrimental because high-resistance L. monocytogenes poses a more dangerous threat to public health (9).

2) In a current study, faster analysis and detection of Listeria species were made possible by an immunoassay “utilizing europium(III)-chelate containing latex nanoparticles as tracers” (7). Each nanoparticle contained 31,000 europium(III)-chelates which improved the labeling activity. “The sensitive nanoparticulate immunoassay developed for Listeria spp. was performed in one-step and two-step formats,” with the former having the advantage of being faster and easier to carry out while the latter was more sensitive (7). This technology is significant because it allows researchers a means to study and detect Listeria species, especially L. monocytogenes in foods, in a more expedient and efficient manner. Of course the nanoparticle assay technology can be modified to detect and study other bacteria (7).

3) A recent study at Purdue University focused on identifying colonies of Listeria species without invasive means. The scientists developed “a non-invasive optical forward-scattering system, called 'scatterometer' for rapid identification of bacterial colonies” (1). The technique is based on the idea that different arrangements of cells in colonies on an agar surface will generate unique forward-scattering patterns because the colonies will vary in size and refractive indices (1). Thus identification of Listeria species can be exacted by taking advantage of known differences in the colonization pattern of each species. For example, using diffraction theory, the researchers were able to identify colonies of Listeria monocytogenes by the specific scattering pattern, radial spokes and rings the colonies made (1). Furthermore, a computer software was developed to extract certain features from the scattering patterns for further analysis. The scatterometer system is accurate 91-100% in identifying different Listeria species. The significance of this technology is that it is simple, non-invasive and does not require any reagent . This allows for scientists to study Listeria without destruction of their colonies (1).


1. Bae, E., Banada, P., Bhunia, A., Bayraktar, B., Guo, S., Hirleman, E., Rajwa, B., and Robinson, J. “Optical forward-scattering for detection of Listeria monocytogenes and other Listeria species”. Biosensors and Bioelectronics. 2007. Volume 22. p. 1644-71. <>

2. Banerjee, S., Cyr, T., Gleeson, T., Hefford, A., and Kalmokoff, M. “Identification of a New Plasmid-Encoded sec-Dependent Bacteriocin Produced by Listeria innocua 743”. Applied and Environmental Microbiology. 2001. Volumbe 67. p. 4041–47. <>

3. Blair, I., Duffy, G., McDowell, D., Sheridan, J. “Development of a surface adhesion immunofluorescent technique for the rapid isolation of Listeria monocytogenes and Listeria innocua from meat. Journal of Applied Microbiology. 1997. Volume 82. p. 225-32. <>

4. Brooks, J., Daneshvar, M., Malcolm, G., Pine, L. “Physiological studies on the growth and utilization of sugars by Listeria species”. Canadian Journal of Microbiology. 1989. Volume 35. p. 245-54. <>

5. Buchrieser, C., Cossart, P., Kunst, F. Glaser, P. and Rusniok, C. “Comparison of the genome sequences of Listeria monocytogenes and Listeria innocua: clues for evolution and pathogenicity”. FEMS Immunology and Medical Microbiology. 2003. Volume 35. p. 207-13. <>

6. Glaser, P., et al. “Comparative Genomics of Listeria Species”. Science Magazine. 2001. Volume 294. p. 849-52. <>

7. Härmä, H., Jaakohuhta, S., Lövgren, T., and Tuomola, M. “Sensitive Listeria spp. immunoassay based on europium(III) nanoparticulate labels using time-resolved fluorescence”. International Journal of Food Microbiology. 2007. Volume 114. p. 288-94. <>

8. Jemmi, T., and Stephan, R. “Listeria monocytogenes: food-borne pathogen and hygiene indicator”. Review Science Magazine. 2006. Volume 25. p. 571-80. <>

9. Li, Q., Logue, C., and Sherwood, J. “Antimicrobial resistance of Listeria spp. recovered from processed bison”. Letters in Applied Microbiology. 2007. Volume 44. p. 86-91. <>